Semiconductor devices such as LEDs and power modules have a structure in which a semiconductor element is bonded on a circuit layer made of a conductive material.
Power semiconductor elements for high-power control that are used to control wind power generation, electric cars, hybrid-power cars, and the like have a high calorific value. Therefore, for example, a power module substrate provided with an insulating layer formed of a ceramic substrate such as aluminum nitride (AlN) or alumina (Al2O3) and a circuit layer formed by providing a metal having excellent conductive properties on one surface of the insulating layer has been widely used as a substrate on which the power semiconductor element is mounted.
On the circuit layer of such a power module substrate, a semiconductor element as a power element is mounted via a solder material (for example, see PTL 1).
As a constituent metal of the circuit layer, aluminum or an alloy thereof, or copper or an alloy thereof is generally used.
Here, in a circuit layer made of aluminum or an alloy thereof, a natural oxide film of aluminum is formed on a surface, and thus it is difficult to satisfactorily perform bonding to a semiconductor element using a solder material.
In a circuit layer made of copper or an alloy thereof, a melted solder material and the copper reacts with each other. The components of the solder material enter the circuit layer and there is a concern that characteristics of the circuit layer may deteriorate.
Accordingly, as shown in PTL 1, a semiconductor element has been bonded using a solder material after formation of a Ni plating film on a surface of a circuit layer.
For example, PTL 2 suggests a technology for bonding a semiconductor element using a Ag nano paste as a bonding method without the use of a solder material.
PTLs 3 and 4 suggest a technology for bonding a semiconductor element using an oxide paste containing metal oxide particles and a reducing agent composed of organic matter.
However, in a case where a semiconductor element is bonded using a Ag nano paste without the use of a solder material as disclosed in PTL 2, a bonding layer composed of the Ag nano paste is made thinner than the solder material, and thus a stress under the load of a thermal cycle easily acts on the semiconductor element, and there is a concern that the semiconductor element itself may be broken.
In addition, also in a case where a semiconductor element is bonded using a metal oxide and a reducing agent as disclosed in PTLs 3 and 4, a thin sintered layer of an oxide paste is formed, and thus a stress under the load of a thermal cycle easily acts on the semiconductor element, and there is a concern that the performance of the power module may deteriorate.
Accordingly, for example, PTLs 5 to 7 disclose a technology for bonding, after formation of a Ag underlayer on a circuit layer made of aluminum or copper using a glass-containing Ag paste, a semiconductor element to the circuit layer via a solder material or a Ag paste. In this technology, by applying and sintering the glass-containing Ag paste on the surface of the circuit layer made of aluminum or copper, the oxide film formed on the surface of the circuit layer is reacted with the glass and removed to form the Ag underlayer, and on the circuit layer on which the Ag underlayer is formed, a semiconductor element is bonded via a solder material.
Here, the Ag underlayer is provided with a glass layer formed by the reaction of the glass with the oxide film of the circuit layer and a Ag layer formed on the glass layer. Conductive particles are dispersed in the glass layer, and conduction of the glass layer is secured by the conductive particles.